Generally, an automatic tool exchange device is used in a machining center or an automatic milling machine in order to automatically exchange a used tool with a new tool provided in a standby position. Such automatic tool exchange device includes a change arm having a gripper shape so as to rapidly and precisely perform the exchange work.
The machining center is a kind of a numerical control machine tool capable of automatically performing various kinds of machining works. The machining center has an automatic tool exchange function and is capable of automatically cutting, drilling or boring at least two planes of a workpiece. In order to achieve such various kinds of machining works in a short period of time, the machining center must automatically exchange tools. Hereinafter, clamping and unclamping operations of a conventional automatic tool exchange device provided in such machining center will be described with reference to FIG. 1.
In a case of a clamping operation, a tool 9 is introduced into a spindle shaft 6 along a tapered surface of the spindle shaft 6, so a collet 10 is inserted into the tool 9. Then, pressurized air is supplied into a pressurized air inlet hole 2 so that a piston 3 and a push rod 6 connected to the piston 3 moved up. As the push rod 5 moves up, a drawbar unit 7 installed in the spindle shaft 6 is also moved up due to elastic bias force of a disc spring 12 provided between the spindle shaft 6 and the drawbar unit 7.
Accordingly, an expansion part 8 formed at an end portion of the drawbar unit 7 is introduced into the collet 10, so that the collet 10 is expanded. Thus, the collet 10 securely grips the tool 9 and the spindle shaft 6, thereby clamping the tool 9.
In a case of an unclamping operation, pressurized air is supplied into a pressurized air inlet hole 1, so the piston 3 is moved down due to pressurized air. As the piston 3 moves down, the push rod 5 coupled to the piston 3 is also moves down along a guide surface of a housing 4.
When the push rod 5 moves down, an end portion of the push rod 5 downwardly pushes the drawbar unit 7 installed in the spindle unit 6, so the drawbar unit 7 is also moved down. As the drawbar unit 7 moves down, the collet 10, which has been expanded, recovers its initial shape and the end portion of the drawbar unit 7 pushes the tool 9, thereby unclamping the tool 9.
Recently, when an automatic tool exchange is installed on a main shaft of a machine tool including a high frequency spindle unit or a belt drive spindle unit, a drawbar unit is provided in a spindle shaft in order to strongly draw a tool or a tool holder including a taper shank, so that the tool or the tool holder is securely fixed to a nose of the spindle shaft. At this time, the drawbar moves in an axial direction of the spindle shaft within a range of 10 mm in order to draw and fix the tool. That is, the drawbar unit can be located in three different positions, such as a front position, a middle position and a rear position, within the range of 10 mm in the axial direction of the spindle shaft depending on a state of the tool.
In the front position, the tool is released from the drawbar unit. Since the tool is in a free state, the tool can be inserted into the spindle shaft or separated from the spindle shaft as required by a user. In the middle position, the drawbar unit securely draws the tool and fixes the tool to the spindle shaft. In the rear position, the tool is separated from the spindle shaft and the drawbar unit returns to its initial position without holding any tools.
Information about three different positions of the drawbar unit must be transmitted to a main controller in order to allow the main controller to prepare a next operation by checking the tool exchange state and in order to reliably and safely carry out machining work. A proximity switch is widely used as an inspection device for obtaining information about three different positions of the drawbar unit. Since only one position may be detected by using one proximity switch, three proximity switches are installed in the spindle unit in order to obtain information about three different positions of the drawbar unit.
However, a size of the proximity switch available from an open market is too large to install the proximity switch in an internal area of the spindle unit. In addition, it is difficult to install three proximity switches within a range of 10 mm corresponding to three different positions of the drawbar unit. Even if three proximity switches are installed in the spindle unit, it is required to adjust the positions of the three proximity switches by using a variable position fixing device so as to precisely install the three proximity switches. Moreover, since three cable lines are necessary for power supply and signal transmission in relation to three proximity switches, a cable line treatment work is additionally required.